1. Field of the Invention
The invention relates to an apparatus and method for forming a scanning electron beam for a cathode ray tube device. More particularly, the invention relates to an apparatus and method for forming a scanning electron beam for a flat screen cathode ray tube device.
Flat screen cathode ray tube devices have been the subject matter of investigations, research and development for many years since a flat screen potentially eliminates the bulk of the envelope of a conventional cathode ray tube, especially a television picture tube. A conventional cathode ray tube, due to the geometry of scanning from an electron gun extending at right angles to the screen, results in a comparatively long neck extending away from the rear portion of the tube. Shortening the neck by moving the gun toward the screen complicates the compensation necessary to eliminate distortion and loss of focus due to the difference in distances between the gun and the center of the screen as compared to the extreme edges of the screen. The length of the neck of a conventional tube is a problem with a small portable set; however, if a truly large screen is desired, for example greater than 30 inches on the diagonal, the bulk of the picture tube resulting from the gun and neck structure becomes overwhelming.
Efforts to develop flat screen tubes to avoid these problems have met with only limited success. Flat screen tubes which are scanned by a single gun or multiple guns emitting electron beams parallel to the surface of the screen have presented complex problems in achieving linear scans with uniform focusing and without distortion. Attempts to use line cathodes and driven grid structures to achieve a limited number of parallel beams for scanning have resulted in extremely complex structures. These problems have been compounded in attempts to provide flat screen picture tubes for providing visual images in color.
2. Description of the Prior Art
Cathode ray tube ("CRT") devices having flat screens are known in the prior art as well as the provisions of elongated or line cathodes in such CRT devices. U.S. Pat. Nos. 4,449,148, 4,451,846 and 4,451,852 each show a flat CRT screen and a plurality of line cathodes. Line cathodes are also shown in U.S. Pat. Nos. 4,429,251, 4,435,672, 4,437,044 and 4,484,103.
The line cathode of U.S. Pat. No. 2,858,464 utilizes perforated grids, the holes of which define the number of electron beams which can be produced from the line cathode. Deflection coils taken with the perforated grids enable a scan of an electron beam to be formed along the length of the line cathode. A middle grid may include a long slit, rather than holes.
In U.S. Pat. No. 3,531,681, emission control grids in conjunction with a slotted emission control plate form a scanning electron beam from the electrons emitted by a line cathode.
In U.S. Pat. No. 4,028,582, there is provided a number of channels or beam guides for deflecting an electron beam from a line cathode in a direction transverse to the beam to produce a segment of a total line scan. Each adjacent channel continues the formation of a total line scan. The beam from the line cathode can be formed in accordance with U.S. Pat. No. 2,858,464.